The present invention relates, in general, to the field of television and video display vertical deflection systems. More particularly, the present invention relates to a universal vertical countdown and method for a video display which will handle all known vertical signals and systems having any number of lines per picture over the range of 513 to 781 lines while providing noise immunity comparable to a dedicated self-locking countdown.
Presently, there are two primary approaches for deriving a vertical pulse in a television or other video display. These two systems are commonly referred to as either injection locked or self-locking systems. In the former injection-locked systems, the vertical pulse is directly locked by the incoming vertical signal at all times. Typically, the vertical input signal is derived from a countdown from the horizontal frequency which, in the United States, is typically 15,734 Hz resulting in a nominal vertical of 525 lines per picture. This injection locked approach will handle both standard and non-standard (free-running vertical), even those in which the horizontal and vertical are not locked, over a somewhat limited range of frequencies. However, its noise performance is relatively poor compared to the alternative self-locking systems.
Certain injection locked systems include both wide and narrow acquisitions modes, the latter mode being utilized to provide improved noise immunity. However, the use of this system does not provide satisfactory operation when applied, for example, in rural areas where the vertical pulse is not derived from the horizontal oscillator but rather from the power source itself. In such instances, the power source may be simply a diesel generator which is nominally run at 50 Hz from whence the vertical pulse is derived. As a result, the supply is generally not accurately controlled and such injection locked systems will maintain operation only in the wide acquisition mode. In this mode, the incoming vertical pulses are monitored to see if they fall within a narrow range of frequencies such that the system can switch to the narrow mode. In those instances in which the vertical pulse is not locked to the horizontal and the vertical frequency is outside of the above narrow range, the injection locked system wou1d remain in the wide acquisition mode and would therefore be more noise sensitive and subject to triggering over a wider range.
Typical self-locking systems utilize an internal countdown to provide improved noise performance but as a result cannot handle some types of non-standard or free-running vertical signals. As with the injection locked systems, the lock range is normally limited to a narrow pulse range to maintain performance. As a consequence, the vertical may drift resulting in picture roll-up and pull-down.
lt wouId therefore be highly advantageous to provide a universal vertical countdown and method ior a video display which can handle both U.S. (525 lines per picture) and European (625 lines per picture) as well as all non standard or free-running vertical signals. In addition, it would be highly desirable to provide a system and method which can lock over a wide range of line systems in addition to those in which the vertical pulse is not derived from the horizontal but rather from a typically poorly regulated power supply.